My plan was India/Bhutan/Nepal/India-Pakistan-Iran-Caucasus-Turkey-etc. As mentioned earlier, you can indeed go to Pakistan overland, at Wagah between Amritsar and Lahore. Today I tried to figure out if you can go from Pakistan to Iran overland.
Here's the deal, fellow armchair travelers! You most certainly can. Take a look at this map and follow along. Cross from Amritsar to Lahore, get yourself to Quetta (about 1/3 way up the Pak/Afghan border); things are reasonable up until there. Then you have to get to Taftan (zoom in one click on the map; it's on the Iran border and called Kuh-i-Taftan on Google Maps) in order to cross to Iran. It's a 24-hour train ride that leaves twice a month, or a 15-16 hour bus ride that leaves daily. This crosses a province called Balochistan. Once you get to Taftan, you cross over to Meer Jawe (/Mirjawa/Mirjaveh), then on to Zahedan. Here are some links that make it seem quite possible.
Here's the other deal! I don't think I will. I'll take a flight to Iran. Balochistan is described as "lawless", "tribal", "dangerous", and "even Pakistan police don't go there." I exaggerate a bit for effect, but I get the feeling it's a notch or two above US-State-Department-dangerous (which is not actually dangerous). You particularly don't want to be there at night, and if the ride takes at least 15 hours, and the border is probably not open at say 8pm, how do you not-be-there-at-night? Furthermore, to be in Iran I have to be part of a tour (because I'm American; more on this later); how do I meet up with them in any reasonable amount of time? (Here are some links that make it seem rather dumb.)
Or, I shouldn't say dumb. I should say, it's certainly possible, and I'd probably be perfectly safe, but I think it's above my threshold of risk. Even more: it's above my investors' threshold of risk. Is that a fair way to put it? I've got a lot going for me, and a lot of people have made it that way. My family, friends, educators, etc. have been investing in me for a while, and by now I'm a figurative pre-IPO Google. They each own a piece of me, and should something terrible happen, it's not just me who goes bankrupt. So avoiding this crossing is, at the very least, a gesture of goodwill towards my metaphorical stockholders.
It's not easy, though! I have a bit of shame about this, a bit of disappointment in missing out on the bus trip of a lifetime through places nobody goes, and a further bit of disappointment that I'll have to say "I went all overland*". I think that the "I want to tell a really badass story about this" angle is foolish, though, so discounting that, the scales tip in favor of flying.
Besides, I'll have a ton of other badass stories to tell!